update changelog for v0.8
update changelog for v0.8

yzhou61/vim-airline,dalguji/vim-airline,HarmtH/vim-airline,chrisbra/vim-airline,tschundler/dotfiles,bling/vim-airline,tschundler/dotfiles,unphased/vim-airline,tschundler/dotfiles,vim-IDE/vim-airline,rchicoli/vim-airline